In the optical fiber cable for use in a superfast optical communications system operating at 40 Gbps or more, comparatively large polarization dispersion occurs, and this becomes a factor for restricting transmission rate or transmission distance. This polarization dispersion occurs as follows. As shown in FIG. 23, degeneration of the base mode occurs due to decentering of a core of an optical fiber cable 100 and application of a non-axisymmetric stress to the core. A group delay difference occurs between a TE wave and a TM wave depending on a difference in the propagation velocity between the optical signals of the two polarized components of the TE wave and the TM wave, which are perpendicular to each other. As a result, broadening occurs in the temporal direction in an optical pulse signal, restricting transmission rate and transmission distance in the communication system. In order to solve the above-mentioned problems, it is required to compensate polarization so that no group delay difference occurs between the two polarized components by controlling the polarization state in the reception terminal station and generating a group delay difference inverse to the group delay difference that occurs in the optical fiber cable.
FIG. 24 is a perspective view showing polarization compensation with a polarization-maintaining optical fiber cable 101 of prior art, and FIG. 25 is a longitudinal sectional view showing a construction of the polarization-maintaining optical fiber cable 101 of FIG. 24.
The prior art example shown in FIG. 24 compensates for the polarization dispersion using the polarization-maintaining optical fiber cable 101. As shown in FIG. 25, this polarization-maintaining optical fiber cable 101 has a predetermined birefringence with a non-axisymmetric stress applied to a core 102 by inserting silica glasses 104 and 105 doped with Ba2O3 on both sides of the core 102 in the portion of a cladding 103 around the core 102. In this polarization-maintaining optical fiber cable 101, the propagation velocities of the polarized components perpendicular to each other are different from each other. Therefore, to the polarization dispersion of the optical fiber cable can be compensated on this basis. In practice, to match the polarization axis of incident light to the polarization axis of the polarization-maintaining optical fiber cable 101, it is required to provide a polarization controller including a half-wavelength plate and a quarter-wavelength plate at the preceding stage of the polarization-maintaining optical fiber cable 101.
FIG. 26 is a plan view showing a construction of the polarization dispersion compensation apparatus disclosed in FIG. 3 of a first prior art document, “Teruhiko Kudou et al., Theoretical Basis of Polarization Mode Dispersion Equalization up to the Second Order, Journal of Lightwave Technology, Vol. 18, No. 4, pp. 614-617, April 2000”. As described above, the polarization dispersion changes depending on environmental changes in the optical fiber cable, and accordingly, there is needed a polarization dispersion compensator capable of adjusting a polarization dispersion value.
In the prior art example of FIG. 26, a polarization beam splitter 111 is provided at the final stage of an optical transmission line 110 for propagating an inputted optical signal, and this is followed by a construction provided with two variable phase shifters PS1 and PS2, a directional coupler DC101, two variable phase shifters PS3 and PS4 and two variable optical delay circuits 113 and 114 as well as a polarization beam combiner 112 further provided at the final stage of the polarization dispersion compensation apparatus. In the polarization dispersion compensation apparatus of this prior art, compensation is made for the polarization dispersion by varying the polarization dispersion by means of an optical processing circuit. However, this prior art example has had the problem that the size of the apparatus has been comparatively large and the transmission loss has been increased due to the use of the bulk type polarization beam splitter and the polarization beam combiner.
FIG. 27 is a plan view showing a construction of the polarization dispersion compensation apparatus disclosed in the specification of U.S. Pat. No. 5,930,414, which is a second prior art document.
The prior art of FIG. 27 is provided with a polarization controller 120 constituted by including a quarter-wavelength plate 121 and a half-wavelength plate 122 provided at the first stage, a polarization beam splitter 123 for splitting an inputted optical signal into two polarized components at the next stage and a polarization beam combiner 124 for combining these two polarized components at the final stage. Moreover, in order to cause a variable delay time between the two polarized components, a plurality of asymmetrical Mach-Zehnder interferometers 130 to 132, which are constituted by including optical waveguides and connected in concatenation, are provided between the polarization beam splitter 123 and the polarization beam combiner 124 via directional couplers 141 to 143, which have adjustable coupling coefficients. Further, it is required to switch the relative optical phase in the Mach-Zehnder interferometers 130 to 132 with respect to the structural common-mode interference of the two optical signals, which are outputted from two arm portions of the Mach-Zehnder interferometers 130 to 132 and thereafter inputted to the directional couplers 141 to 143 at the subsequent stages. For the above reasons, the Mach-Zehnder interferometers 130 to 132 are provided with variable phase shifters 150 to 152, respectively. Therefore, in this prior art example, compensation is made for the polarization dispersion by combining the directional couplers 140 to 143 with an optical processing circuit that has a Mach-Zehnder structure. However, even this prior art example has had the problem that the size of the apparatus had become comparatively large. Therefore, the polarization dispersion compensator is required to have a lower loss and a small size as well as a high operating speed and a low consumption of power.
FIG. 28 is a plan view showing a construction of the polarization dispersion compensation apparatus disclosed in a third prior art document, “Japanese Patent Laid-Open Publication No. 2001-42272”.
In the prior art of FIG. 28, an input channel optical waveguide 211, an optical waveguide type polarization beam splitter element 212, a pair of optical waveguides 213a and 213b, a variable branching ratio optical coupler 214, a pair of optical delay lines 215a and 215b, an optical waveguide type polarization beam combiner element 216 and an output channel optical waveguide 217a are successively formed on a silicon substrate 210. Further, polarization change means 218a and 218b are formed in either one of a pair of optical waveguides 213a and 213b and in either one of a pair of optical delay lines 215a and 215b. Moreover, phase adjustment means 219a and 219b for adjusting the relative phase difference are formed in a pair of optical waveguides 213a and 213b. In this case, one input port of the optical waveguide type polarization beam splitter element 212 is optically connected with the input channel optical waveguide 211, while two input ports of the variable branching ratio optical coupler 214 are optically connected with the two output ports of the optical waveguide type polarization beam splitter element 212 via a pair of optical waveguides 213a and 213b. Moreover, a pair of optical delay lines 215a and 215b has one end optically connected with two output ports of the variable branching ratio optical coupler 214 and the other end optically connected with two input ports of the optical waveguide type polarization beam combiner element 216. One output port of the optical waveguide type polarization beam combiner element 216 is optically connected with the output channel optical waveguide 217a. 
In the polarization dispersion compensation apparatus constituted as above, the phase adjustment means 219a and 219b and the variable branching ratio optical coupler 214 operate as a polarization controller 300, and the optical delay lines 215a and 215b operate as a variable delay line, constituting the polarization dispersion compensation circuit as a whole. With this arrangement, this polarization dispersion compensation circuit can compensate for the primary polarization dispersion in the optical transmission line.
In this polarization dispersion compensation apparatus, the polarization controller 300 exists between the polarization beam splitter element 212 and the optical delay lines 215a and 215b. An optical signal inputted to the polarization beam splitter element 212 is split into the TE wave and the TM wave. Subsequently, the optical signal has a polarization controlled by the polarization controller 300 constituted by including the phase adjustment means 219a and the variable branching ratio coupler 214, and thereafter, the controlled optical signal is inputted to a pair of optical delay lines 215a and 215b. The amount of delay of the optical signal is adjusted by the optical delay lines 215a and 215b, and the polarization dispersion of the optical signal is compensated. However, in this polarization dispersion compensation apparatus control of compensator of the polarization dispersion is very difficult since it is required to adjust the three points of the phase adjustment means 219a and 219b, the variable branching ratio optical coupler 214, and the optical delay lines 215a and 215b. 
It is an object of the present to solve the aforementioned problems and provide a polarization dispersion compensation apparatus, which has a smaller size and a lighter weight than the prior art, and is able to compensate for the polarization dispersion with lower loss.
Another object of the present invention is to solve the aforementioned problems and provide a polarization dispersion compensation apparatus, which is able to control compensation for a polarization dispersion more easily than in the prior art.